1. Field of the Invention
This invention relates to magnetic circuits for accelerometers and, more particularly, to a method for improving the scale factor linearity of a servo accelerometer.
2. Prior Art
A servo accelerometer is, in effect, an electromagnetically operated servo motor which includes a force restoring element or force balancing element, a coil of wire in a pendulous device called a torque coil, which is immersed in a magnetic field. The torque coil is fixed to a moving element, or so-called sensing element, which responds to external acceleration forces applied to the accelerometer. The accelerometer has a magnetic circuit which includes elements such as a permanent magnet source and other magnetic circuit components which serve as pole pieces or which provide a return path for the magnetic flux in the circuit. The torque coil is immersed in the magnetic field provided by the permanent magnet. Current through the torque coil interacts with the magnetic field of the permanent magnet, producing a force on the torque coil. The deviation of the flexure element from a neutral position is sensed by a pickoff device and a current proportional to the deviation is produced by an external circuit. The current is set to provide a force which balances those forces on the moving element of the accelerometer caused by acceleration. Consequently, the current through the torque coil is very nearly proportional to the force on the sending element and to the acceleration to be measured. The current through the torque coil is easily converted to a suitable signal indicating the magnitude of the acceleration. A servo accelerometer of this type is disclosed in the Woodruff U.S. Pat. No. 4,779,463, issued to the assignee of the instant invention.
Use of this type of accelerometer is not without problems. In this type of accelerometer, the force caused by the current in the torque coil is proportional to the cross product of the current through the coil and a magnetic field. The magnetic field is produced by combining two magnetic fields: One field is the permanent magnet field. The other field is the magnetic field caused by the current through the torque coil. The magnetic field produced by the current in the torque coil is a nonlinear function of the current in the torque coil because the field produced by the coil current interacts with the field from the magnet and magnetic return path in a nonlinear way. This results in the current through the torque coil being a nonlinear function of the acceleration being measured.
It is possible to produce a design for an accelerometer which theoretically has a nearly linear scale factor. However, production variations normally result in small amounts of nonlinearity. For certain applications, such as inertial navigation, it is desirable to reduce these small nonlinearities even further to meet certain critical precise performance specifications.